Free piston construction



April 7, 1959 c. R. SAKRAIDA ErAL 2,

FREE PISTON CONSTRUCTION Filed May 23, 1957 ATTORNEY United States -site FREE PISTON CONSTRUCTION Carl R. Sakraida, Downers Grove, and Robert H. Wellman, Clarendon Hills, 111., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application May 23, 1957, Serial No. 661,089

7 Claims. (Cl. 123-46) This invention relates to free piston engines and more particularly to a compressor and power piston assembly for such an engine.

The invention is concerned with free piston engines of the type that have a piston assembly that comprises an annular compressor piston upon which is coaxially secured a hollow diesel piston of smaller diameter, the piston assembly being reciprocally mounted in adjacent and coaxial compressor and diesel cylinders. A pair of symmetrically opposed piston assemblies are usually used and the piston assemblies have a synchronizing connection to keep them mechanically phased. Cushion chambers are located at the outboard ends of the piston assemblies and a pneumatic connection is provided between the chambers to keep the piston assemblies pneumatically phased. The pneumatic connection traverses the compressor pistons and the synchronizing connection attaches to the compressor pistons thus restraining the assemblies against rotation. I

The cylinder axis of an engine of this type is horizontally disposed so that gravity forces acting on the moving piston assemblies are equalized and the weight of the assemblies is accordingly borne by the lower half of the cylinders. Since the piston assemblies are restrained against rotation, wear is concentrated on the lower half of the assemblies and especially on the lower half of the small diameter diesel or power pistons which, accordingly, requires replacement oftener than the compressor piston. The power piston also operates under more severe temperature and lubrication conditions than the compressor piston and this further necessitates more frequent replacement.

An object of the invention is to provide a piston assembly for free piston engines that will extend the life of the power piston.

Another object of the invention is to provide a piston assembly for free piston engines wherein the power piston may be readily detached from the compressor piston for replacement purposes.

The invention accomplishes these objects by providing a construction wherein the power piston is detachably mounted on the compressor piston by an internal snap ring, wherein the power piston is provided with a constant flow of oil for cooling and lubrication and wherein the power piston is provided with an anti-friction bearing band to reduce wear. I

Further objects and advantages of the present invention will be apparent from the following description, reference being bad to the accompanying drawing, wherein a preferred form of the present invention is clearly shown.

In the drawing:

Figure 1 is a vertical section of the left hand portion of a free piston engine in accordance with the invention; and

Figure 2 is an enlarged section of the diesel piston por- I tion of Figure 1. 7

Referring to Figure 1, the free piston engine is of the gasifier type and only the left hand portion is shown as the right hand portion is similarly constructed and as this type of engine is well known in the art. The engine has a central engine case 10 which mounts a central diesel cylinder 12, the case and cylinder forming a scavenge air chamber 14 that supplies compressed air through diesel intake ports 16 to the combustion chamber 17. A compressor cylinder 18 and a cushion chamber cover 20 are mounted at the end of the engine case 10 to form a compressor chamber 22 and a cushion chamber 24. The right hand portion of the engine, not shown, is generally similar to the left hand portion, as previously noted.

An annular compressor piston 26 having split compression rings 28 is reciprocal in the compressor cylinder 18 and mounts a smaller diameter annular sleeve 29 by bolts 30, the sleeve having a detachable connection, to be described, with a hollow diesel piston 32 that is reciprocal in the diesel cylinder 12.

An annular row of suction valves 34 are located at the inboard end of the compressor cylinder 18 and a valve plate 36, mounting a ring of delivery valves 38, is mounted between the engine case 10 and the compressor cylinder 18 to separate the compressor chamber 22 from the scavenge chamber 14. A cushion chamber balance tube.

40 extends from one end of the engine to the other to place the left and right cushion chambers 24 in communication with each other and thereby balance the engine pneumatically. The balance tube 40 passes through each valve plate 36 through each compressor piston 26 and restrains the compressor pistons against rotation in the compressor cylinders. The balance tube 40 is fixed to the valve plates 36 but is slidably related to the compressor pistons 26. A synchronizing bar 42 is secured to the left compressor piston 26 and a like bar is connected to the right compressor piston. A conventional synchronizer mechanism, not shown, connects the bars 42 to mechanically phase the left and right piston assemblies.

A cooling oil supply conduit 44 connects with an oil spray pipe 46 and a cooling oil return conduit 48 connects with an oil drain pipe 50, the pipes being secured to the cushion chamber cover 20 so as to project therefrom in spaced, concentric relation. An oil delivery tube 52 and an oil return tube 54 are secured to the diesel piston 32 in spaced, concentric relation for movement therewith and are telescopically connected with the respective fixed pipes 46 and 50.

The spray pipe 46 has a nozzle 56 that delivers a steady stream of cooling oil to the delivery tube 52, and the interior of the diesel piston 32 is bathed with the oil which is then exhausted by the return tube 54.

The engine operates in the well known manner, the left and right compressor pistons 26 furnish compressed air to the central scavenge chamber 14, the scavenge chamber supercharges the central combustion chamber 17, and the left and right cushion chambers act as air springs to absorb energy on the outward power strokes of the piston assemblies to return them for the inward compression strokes. Fuel is supplied to the combustion chamber 17 in the usual fashion and the high pressure, high temperature exhaust from the engine is expanded through a gas turbine, not shown, to provide shaft horsepower.

Since the axis of the engine is horizontally disposed, the weight of each piston assembly is borne by the lower half thereof. The balance tube 40 and synchronizing bars 42 restrain the piston assemblies from rotating and the lower portions of the assemblies are therefore susceptible to excessive wear in certain'locations. Thecompressor pistons 26 are of large diameter and accordingly lems. Wear problems due to gravity do arise, however, in conjunction with the diesel pistons 32 because of their Pasta pr- 7. .59

small diameter. An anti-friction bearing band 58, preferably of sintered iron or bronze, is therefore located on each diesel piston 32 to support the inward ends of the piston assemblies in the diesel cylinder 12.

Referring now to Figure 2, the diesel piston 32 is preferably made of cast iron and includes a crown portion 60 and spaced outer and inner skirt portions 62 and 64 which form an annular oil splash chamber 66. A wall 68 secures the forward end of the oil delivery tube 52 and forms an oil supply chamber 70 for the diesel piston. Radial openings 72 place the supply chamber 70 in communication with the splash chamber 66 and radial openings 74 place the splash chamber 66 in communication with an oil discharge chamber 78 formed by the inner skirt portion 64. Oil flow through the interior of the diesel piston occurs by way of the tube 52, the chamber 70, the openings 72, the chamber 66, the openings 74, the chamber 78 and the tube 54. The oil flow is preferably of a lesser degree than would completely fill the interior of the diesel piston during engine operation. Eflicient cooling is achieved without filling the piston because the reciprocating movement violently splashes and dashes the oil about in the piston interior insuring complete bathing thereof.

The annular sleeve or trunk 29 of the compressor piston 26 has a ring 79 secured to its inward end by a weld 80 to form an inwardly flanged shoulder which provides a radial thrust surface at 81 and a radial locking surface at 82. The diesel piston skirt portions 62 and 64 join at 83 to provide a radial thrust surface at 84 and a cylindrical flange 86 which is peripherally grooved to receive a snap ring 88 which is readily accessible through the interior of the trunk 29 to remove the power piston 32. The bearing band 53 is preferably oil pervious and includes a sleeve portion 90 that has a press fit at 1% on the reduced diameter skirt portion 92 and also includes a disk portion 94 that is interposed between the trunk shoulder 79 and skirt juncture 83 to provide radial thrust surfaces at 81 and 84.

The antifriction bearing band 58 is provided to take up the primary wear on the diesel piston. The disk portion 94 acts as an anti-friction washer to transmit axial thrust between the compressor piston and diesel piston and will permit the diesel piston to be manually rotated with respect to the compressor piston which, as previously noted, is restrained against rotation. The diesel piston is installed on the sleeve 29 by inserting the flange 86 through the shoulder 79 and thereafter springing the split snap ring 88 into the groove in the flange.

The diesel cylinder 12 is preferably of cast iron to provide a complementary running surface for the piston 32. The bearing band 58 is sintered with sufficient porosity to pick up oil from the splash chamber 66 by way of radial passages 96 in the outer skirt 62 and thereby lubricate the running surface 98. A pair of split oil wick rings 102, which may be of sintered metal, are carried in peripheral grooves in the bearing band 58 and a plurality of grime collector grooves 104 encircle the hearing hand between the oil Wick rings. Split oil control or scraper rings 106 are located at each end of the bearing band 58 in respective peripheral grooves in the bearing band and sleeve shoulder 79. This oil ring arrangement insures an adequate supply of oil at the major gravity wear surface 98 of the engine. The inward end of the outer piston skirt 62 is peripherally grooved to receive split compression rings 108.

Bolts 110 secure the return tube 54 to a flange 112 that extends from the skirt juncture 83 and a cylindrical battle 114 is also secured to the flange. The baflie 114 blocks off a portion of the oil discharge openings 74 and may be made in various sizes to regulate the quantity of oil in the splash chamber 66. Any specific engine can have a baflle 114 of a specific size that is adapted for the normal condition of engine operation or the openings 74 can a be predetermined to correct size for a specific engine and the baflie eliminated.

The power thrust load between the compressor and diesel piston is, of course, in an axial direction and is compressive in nature since the piston assembly is subject to the combustion chamber pressure at one end and to the cushion chamber pressure at the other end. The inner and outer skirt portions 64 and 62 join together to bear against the disk portion 94 of the bearing band 58 and the compressive thrust in the diesel piston is accordingly divided substantially equally between the skirt portions thus permitting the outer skirt 62 to be relatively thin for good heat dissipation.

While the preferred embodiment of the invention has been described fully in order to explain the principles of the invention, it is to be understood that modifications at structure may be made by the exercise of skill in the art within the scope of the invention which is not to be regarded as limited by the detailed description of the preferred embodiment.

We claim:

1. In a free piston engine, a compressor cylinder, a power cylinder, an annular compressor piston reciprocal in the compressor cylinder and restrained against rotation, a hollow power piston reciprocal in the power cylinder, the pistons having cooperating bearing surfaces upon which one piston is freely rotatable with respect to the other piston, means mechanically retaining the pistons together without preventing relative rotation therebetween, and means for lubricating the bearing surfaces.

2. In a free piston engine, a compressor cylinder, a power cylinder, an annular compressor piston reciprocal in the compressor cylinder and restrained against rotation, a hollow power piston reciprocal in the power cylinder, the pistons having cooperating bearing surfaces upon which one piston is freely rotatable with respect to the other piston, a snap ring within the interior of the pistons mechanically retaining the pistons together without preventing relative rotation therebetween, and means for lubricating the bearing surfaces.

3. A piston assembly for a free piston engine comprising a compressor piston having a projecting annular trunk, the trunk being formed with an inwardly flanged shoulder presenting a radial surface, a power piston having a crown portion and having a skirt portion depending therefrom to form a radial surface engaging the radial surface of the trunk shoulder, a cylindrical flange portion extending from the skirt portion and received within the trunk shoulder and having an external peripheral groove, and a snap ring received in the peripheral groove and abutting the trunk shoulder to retain the compressor piston and the power piston in assembled relation.

4. A piston assembly for a free piston engine comprising a compressor piston having a projecting annular trunk, the trunk being formed with an inwardly flanged shoulder presenting a radial surface, a diesel piston having a crown portion and having spaced inner and outer skirt portions depending therefrom and connected to each other to form a radial surface engaging the radial surface of the trunk shoulder, a cylindrical flange portion extending from the juncture of the skirt portions and received within the trunk shoulder and having an outer peripheral groove, and a snap ring received in the peripheral groove and abutting the trunk shoulder to retain the compressor piston and the diesel piston in assembled relation.

5. A piston assembly for a free piston engine comprising a compressor piston having a projecting annular trunk, the trunk being formed with an inwardly flanged shoulder presenting a radial surface, an antifriction wear band having a disk portion presenting radial surfaces on opposite faces, one of which engages the trunk shoulder radial surface, and having a sleeve portion presenting inner and outer cylindrical surfaces, a diesel piston having a crown portion and having spaced inner and outer skirt portions depending therefrom and connected to each other to form a radial surface engaging the other radial surface of the disk portion, the outer skirt portion having a cylindrical bearing surface press fitted in the inner cylindrical surface of the sleeve portion, a cylindrical flange portion extending from the juncture of the skirt portions and received within the trunk shoulder and having an outer peripheral groove, a snap ring received in the peripheral groove and abutting the trunk shoulder to retain the compressor piston and the wear band and the diesel piston in assembled relation, the inner and outer skirt portions forming an annular oil splash chamber extending from the crown portion to the juncture of the skirt portions, the outer skirt portion having radial oil passages formed therein extending from the oil splash chamber to the inner cylindrical surface of the sleeve portion, the outer cylindrical surface of the sleeve portion having peripheral oil ring grooves and grime collector grooves formed therein, and oil rings in the oil ring grooves.

6. In a free piston engine, a compressor cylinder, a power cylinder of smaller diameter than the compressor cylinder and arranged in adjacent and coaxial relation therewith, a compressor piston reciprocal in the compressor cylinder and providing a generally radial shoulder, a power piston reciprocal in the power cylinder having a crown portion and a skirt portion depending therefrom to form a generally radial shoulder, a wear band of L-shaped section providing a disk-shaped thrust portion engaged between the shoulders and providing a sleeve-shaped wear portion encircling the shoulder of the skirt portion, and means for securing the power piston to the compressor piston.

7. In a free piston engine, a compressor cylinder, a diesel cylinder of smaller diameter than the compressor cylinder and arranged in adjacent and coaxial relation therewith, the cylinders being horizontally disposed, an annular compressor piston reciprocal in the compressor cylinder, an annular sleeve fixed to the compressor piston at one end and reciprocal in both cylinders, the sleeve having an inwardly flanged generally radial shoulder at the other end, a diesel piston reciprocal in the diesel cylinder having a crown portion and a skirt portion depending therefrom to form a generally radial shoulder and to form an axially extending cylindrical flange received within the sleeve shoulder, and a snap ring locating the skirt flange axially on the sleeve shoulder.

References Cited in the file of this patent UNITED STATES PATENTS 2,645,213 Huber July 14, 1953 FOREIGN PATENTS 830,127 Germany Jan. 31, 1952 

